Preloaded bolts and external tension

To understand the effect of applied external tension, let us first of all consider the initial state of preloaded bolt arrangement without the external force.

Figure 1 below shows the equilibrium of components in the initial state. In preloaded bolts, the tension induced into the shank of the bolt is balanced by the compression force under the head and nut of the bolt, which in turn induces pressure at the faying surfaces.

Figure 1.

Let us now consider the state with an external force. It should be noted that external tension is not applied directly on the bolt but rather on the plates in the joint. As a result of that, the applied tension reduces the intensity of contact force (compression force) at the faying surfaces and hence the frictional resistance of the connection is lower. The tension in the bolt does not change unless it exceeds the preload force. After the exceedance of that force, the plies separate and the connection behaves as a non-preloaded joint with the force in the bolt equal to the applied external tension.

Please note that where prying forces occur, these should be included in the value of the external force.